1. Field of the Invention
The present invention relates to a method for producing carbamates and a method for producing isocyanates by using the carbamates produced by the method for producing carbamates.
2. Description of the Prior Art
Alkyl carbamates are organic compounds useful as raw materials of medicines, agricultural chemicals and the like; raw materials of a variety of fine chemicals; reagents for analysis of alcohols; and industrial raw materials used for various purposes.
A variety of methods for producing alkyl carbamates are known, including (1) a method of reacting isocyanate with alcohol, (2) a method of reacting chloroformate ester with amine in the presence of base, (3) a method of reacting phosgene with alcohol and amine, (4) a method of reacting urea with alcohol, (5) a method of reacting dimethyl carbonate with formamide, and (6) a method of reacting dialkyl carbonate with amine.
In recent years, many studies have been made for the uses of the alkyl carbamates as raw materials for producing isocyanates without using any phosgene.
Isocyanates is an organic compound containing an isocyanate group and is in wide use as raw materials of polyurethanes. Isocyanates is produced industrially by reacting amine with phosgene (Phosgenation). However, phosgene is highly toxic, corrosive and inconvenient in handling. In recent years, as an alternative to the phosgenation, a variety of methods for producing isocyanates economically have been proposed, according to which after carbamates are prepared from amine by using dialkyl carbonate, the carbamates obtained are thermally decomposed to thereby produce isocyanates (e.g. Japanese Laid-open (Unexamined) Patent Publications No. 7(1995)-165696, No. 6(1994)-172292, and No. 9(1997)-249633).
The methods for producing the alkyl carbamates cited above have the following disadvantages, however. The method (1) has the disadvantage that an irritating isocyanate must be handled as raw material and accordingly careful handling is required. The method (2) has the disadvantage that a base of a number of moles equal to or more than equimolar must be used. The method (3) has the disadvantages that phosgene is highly toxic and corrosive and that a base must be used in the reaction. The methods (4) and (5) have the disadvantage that the reaction must be done at high temperature or high pressure.
In the method (6), dimethyl carbonate is used as dialkyl carbonates (which is well known from Japanese Patent Publication No. Sho 51 (1976)-33095, Japanese Laid-open (Unexamined) Patent Publication No. 57(1982)-82361, and U.S. Pat. No. 4,395,565, for example). In this method, dimethyl carbonate is allowed to react with amine in the presence of a Lewis-acid, a lead, titanium or zirconium catalyst, or an alcoholate of alkali metal or alkali earth metal. The examples of this method show, however, that the rate of reaction is generally slow and also a N-methyl compound as a by-product is easily produced by the reaction. Due to this, the method (6) has the disadvantage that it is hard to provide improvement in space time yield (STY) of carbamates.
On the other hand, for example, Japanese Laid-open (Unexamined) Patent Publication No. Sho 64(1989)-85956 describes, as a method which suppresses the N-methylation reaction, the method by adding amine and alcoholate catalyst of alkali metal or alkali earth metal continuously or intermittently.
However, since this method requires a large amount of alcoholate catalyst for producing carbamates at high yields, production costs increase unavoidably. In addition, since the catalyst is neutralized, a large amount of salt is produced and resultantly a considerable burden is put on the recovery and purification of carbamates.
Further, an alternative method using carbonates has been proposed, for example, by Japanese Laid-open (Unexamined) Patent Publication No. Hei 6(1994)-128215 which teaches the method of producing an alkyl carbamate from alkyl aryl carbonate and aromatic amine. In this method, a nitrogen-containing heterocyclic compound is used as the catalyst. However, this method has the disadvantage that a large amount of the catalyst is required and improvement in yield cannot be attained.
In the methods for producing isocyanates previously mentioned, after carbamates are prepared from amine by using dialkyl carbonate, the carbamates obtained are thermally decomposed. However, as described, for example, in the examples of Japanese Laid-open (Unexamined) Patent Publication No. Sho 64(1989)-85956, when a base catalyst is heated together with carbamates, carbamates are changed further and are changed into an unintended high boiling material. Due to this, the neutralization process is unavoidable. As a matter of fact, prior to the thermal decomposition process of carbamates, it is necessary that the residual base catalyst in carbamates is neutralized by adding a phosphoric acid and the phosphoric acid excessively added is eliminated by washing.